Pattering method

ABSTRACT

The invention provides a method of pattering a substrate, in which a first material in solution is deposited on the substrate. The composition of the solution of the first material is selected so it dries to leave a residue of the first material on the substrate, the residue comprising a thin film in the centre and a ridge around the perimeter. The residue is etched to remove the thin film, leaving the ridge on the substrate. After etching the ridge is hydrophobic and the substrate is hydrophilic. An aqueous solution of a second material is then deposited on both sides of the ridge. After the aqueous solution has dried, the ridge is removed, leaving a layer of the second material on the substrate, the layer having a narrow gap therethrough. The layer may be used for the source and drain electrodes of an organic thin film transistor.

[0001] The present invention relates to a method of patterning asubstrate. In particular, the method of the present invention relates topatterning substrates to provide electronic devices such as thin filmtransistors (TFTs) and/or electro-optic devices thereon.

[0002] Photolithography is presently widely used for the mass productionof electronic devices and achieves very high resolution andregistration. In photolithography, a spin-coated photoresist layer isprovided on a substrate and is exposed by blue or ultraviolet light withan aligner or stepper, which aligns patterns on a master, comprising amask or reticule, with the substrate. The exposed photoresist is thendeveloped to provide patterns of the photoresist on the substrate. Thisis generally followed by an etching or deposition process to pattern anobjective material. The resolution achieved by photolithography isdetermined by the wavelength of the exposure light and the optics of thealigner or stepper.

[0003] Presently, this photolithographic technique is used not only forsmall-sized integrated circuits but also for very large active-matrixdisplays. For example, thin film transistor (TFT) arrays foractive-matrix liquid crystal display (LCD) panels require substrateslarger than 50 cm². Particularly high resolution and registration arerequired for producing arrays of TFTs in LCD panels, since the channellengths of the TFTs should ideally be lower than 20 μm. However, it hasbeen found that such large substrates tend to exhibit bending,presenting difficulties in providing sufficiently accurate resolutionand registration. Furthermore, the photolithographic process must beperformed several times to make a complete device and this presentsfurther difficulties in repeating registration with sufficient accuracy.However, manufacturers usually use a single aligner, having sufficientlyhigh resolution and very precise registration mechanism, not only forthe formation of channels but also for the other patterning steps. Suchan alignment system is expensive. Moreover, processes using such analigner are also expensive, thereby raising the manufacturing cost ofLCD panels.

[0004] In order to reduce the cost entailed by the use ofphotolithography, a variety of non-photolithographic patterningprocesses have been proposed. For example, micro-contact printing andmicro-moulding techniques have been found to be capable of patterningfeature sizes down to 1 μm. These techniques use elastic rubber stampsfor printing so as to provide good contact between the stamp and asubstrate. Due to its elasticity, however, the stamp becomes distorted,which makes it difficult to align the master stamp with the patterns ona substrate. Thus, these techniques have the significant disadvantage ofdifficulties in accurate registration, especially with large substrates,such as those used for LCD panels.

[0005] Inkjet printing technology is now used widely for personalprinting. It achieves a very high quality of print, approachingphotographic grade. Inkjet printing has also proved to be a promisingtechnique for manufacturing electronic devices such as colour filtersfor liquid crystal displays and full-colour electroluminescent displays.To achieve such electroluminescent displays, different conjugatedpolymers are deposited using an inkjet printing technique to providethree colours (blue, green, and red) in the display.

[0006] Inkjet techniques had previously been regarded as beingcomparatively low-resolution patterning techniques and it had thereforepreviously been thought that inkjet printing was unsuitable forproducing TFTs. This is because organic polymer TFTs require a channellength of less than 20 μm to achieve a sufficiently high drain current.To produce such a TFT using an inkjet technique, source, drain and gateelectrodes must be printed on a substrate. The source and drainelectrodes must have a very small gap between them, since this gapdefines the channel in the TFT. Since polymer semiconductors have lowcarrier mobility, this gap should be less than 20 μm, as noted above, inorder to achieve practical characteristics.

[0007] However, the resolution presently achieved simply by inkjetprinting on a solid substrate is not sufficiently high to pattern thesource and drain electrodes with a suitably small gap therebetween(channel length), due to fluctuations in the printing process. Inparticular, the direction of flight of ink droplets is not alwayscompletely perpendicular to the face of the nozzle plate of the inkjetprint head from which they are ejected, resulting in patterning errors.Furthermore, an ejected droplet spreads on the surface of the substrateonto which it is ejected. The amount the droplet spreads is a functionof the surface energies and the interfacial energy of the solidsubstrate and of the liquid droplet respectively. However, there arefluctuations in the surface energy and the interfacial energy of thesolid surface. This results in variations in size of respective dropletsdeposited on the substrate. Accordingly, the width of the gap betweentwo deposited droplets and hence the channel length of a printed TFT isvariable and, in the worst case, short circuits are formed between thesource and drain electrodes.

[0008] Nonetheless, all-polymer TFTs have previously been fabricated byinkjet deposition. In such fabrication, the source, drain and gateelectrodes are formed of a conducting polymer, PEDOT(poly-ethylenedioxythiophene, Baytron P from Bayer AG), and depositedusing an inkjet technique. In order to obtain a satisfactory channelwidth, inkjet printing can be combined with the pre-patterning ofwetting properties. This allows control of the flow of ink on thesubstrate by using a pattern of hydrophilic and hydrophobic substrateregions. As shown in FIG. 8(b), a non-wetting or hydrophobic repellingstrip 102 of polyimide (PI) can first be formed on a glass substrate 100by photolithography, micro-contact printing, micro-moulding printing orphoto-induced wettability patterning. This repelling strip 102 definesthe channel 106 of the TFT, the width of the strip 102 being the lengthL of the channel 106, as shown in FIG. 8(a). The remaining area of thesubstrate 100 is hydrophilic or wetting with respect to a solution ofPEDOT. Source and drain electrodes can then be formed by depositing awater-based solution of PEDOT onto the glass substrate using an inkjetprint head. The PEDOT solution exhibits relatively high contact anglesof around 70° on the PI strip, and small contact angles of less than 20°on the glass region. Thus, when droplets 104 of PEDOT solution aredeposited along the strip 102, the droplets 104 spread over thesubstrate 100 but are repelled by the strip 102. The solution 104 on thesubstrate 100 is therefore confined from spreading over the repellingstrip 102, but instead aligns along the side of the strip 102. Usingthis self-aligning mechanism, source 108 and drain 110 electrodes with achannel length L shorter than 20 μm and as low as 5 μm can be achieved.

[0009] In practice, photolithography has previously been used to formthe PI strip. However, the use of lithography entails many stepsincluding the application of a primer, coating of a photoresist,mask-alignment, exposure, baking, development, etching and stripping.The use of lithography therefore significantly increases process costsand outweighs the benefits of inkjet printing. Micro-contact printingand micro-moulding printing with an elastic stamp (mould), for examplemade of PDMS (polydimethylsiloxane) could also be used forpre-patterning, but the distortion and deformation of the elastic stampdiscussed above deteriorates accuracy in registration. As analternative, the recently developed method of photo-induced wettabilitypatterning appears promising since it can be expected to involve asmaller number of steps than lithography. However, this method has notyet been properly established. In particular, in the current state ofthe art of this method, the wavelength of the light is too short and thesensitivity of the method is too low for realistic application.

[0010] Thus, there is no suitable technique for consistently providingsufficiently small channel lengths in a cost effective manner.

[0011] According to a first aspect of the present invention, there isprovided a method of patterning a substrate, the method comprising:

[0012] depositing a first material in solution on the substrate, whereinthe solution of the first material is selected such that a profile ofthe first material dried on the substrate includes a ridge;

[0013] etching for removing portions of the first material such that theridge of the first material remains on the substrate; and

[0014] depositing a second material in solution on or adjacent the ridgeof the first material.

[0015] Embodiments of the present invention will now be described by wayof further example only and with reference to the accompanying drawings,in which:

[0016]FIG. 1 illustrates a method for patterning a substrate inaccordance with the present invention;

[0017]FIG. 2 shows plan views of a substrate during patterning using themethod illustrated in FIG. 1;

[0018]FIG. 3 shows cross-sections of material deposited in solution on asubstrate and subsequently dried;

[0019]FIG. 4 illustrates fluid flow within a droplet of a solution asthe solvent evaporates;

[0020]FIG. 5 shows etching of a substrate in accordance with the presentinvention;

[0021]FIG. 6 also shows etching of a substrate in accordance the presentinvention;

[0022]FIG. 7 illustrates deposition of a droplet of a solution on thesubstrate shown in FIG. 6;

[0023]FIG. 8 shows a prior art pre-patterned contrast in wettability;

[0024]FIG. 9 shows a thin film transistor fabricated in accordance withthe present invention; and

[0025]FIG. 10 shows a method of fabricating a plurality of thin filmtransistors in accordance with the present invention;

[0026]FIG. 11 shows a block diagram of an electro-optical device;

[0027]FIG. 12 shows a schematic view of a mobile personal computerincorporating a display and other devices fabricated in accordance withthe present invention;

[0028]FIG. 13 shows a schematic view of a mobile telephone incorporatinga display and other devices fabricated in accordance with the presentinvention; and

[0029]FIG. 14 shows a schematic view of a digital camera incorporating adisplay and other devices fabricated in accordance with the presentinvention.

[0030] The present invention provides a high-resolution patterningtechnique that does not require photolithography or similar processes.Briefly, in one aspect of the present invention pre-patterning is formedwith a first material, which is deposited on a substrate in the form ofdroplets of a solution of the first material such as, for example, anorganic polymer dissolved in a solvent. The use of an inkjet print headhas been found to be suitable for this purpose. Droplets deposited onthe substrate spread, the degree of spreading being determined by theparameters discussed above. However, the solution comprising the firstmaterial is formulated such that the perimeter of a deposited dropletremains constant with respect to the substrate as solvent evaporates andthe droplet dries to leave a residue of the first material. This will betermed hereafter pinned contact line deposition and will be explained inmore detail below. The pinned contact line deposition results inring-shaped deposition of the first material, in which most of thesolute in the droplet gathers on the contact line and is depositedthere. Thus, the dried residue of the first material comprises a ridgedisposed around the edge and a thin film in the centre region. The ridgehas a narrow width compared with the diameter of the droplet and thiswidth can be controlled. The width of the ridge can be controlled by theviscosity of the solution or by the drying speed of the solution. Theviscosity of the solution is changed by the concentration of organicpolymer material in solution. Therefore the width of the ridge can becontrolled by controlling the concentration of the organic polymersolution. The viscosity of the solution, of course, depends additionallyon the viscosity of the solvent. As will become apparent from thedescription below, a relatively low concentration and thereforerelatively thin solution will give rise to a relatively narrow ridge andthis characteristic can be used to advantage in the method of thepresent invention. Such a narrow ridge is also obtained by using lowviscosity solvent. Furthermore, a narrow ridge can be achieved if thesolution is dried relatively quickly by using a volatile solvent, byproviding a gas flow over the solution, which may be a heated gas flow,or by heating the substrate onto which a droplet has been deposited.Naturally, any combination of the above techniques may be used tocontrol the drying speed and/or the viscosity of the solution.

[0031] This deposition step is followed by wet or dry etching in orderto remove the thin film in the centre region and to leave a narrow ridgeon the substrate. The width of the ridge can be further controlled bythe etching process since the cross-sectional shape of the ridge issubstantially triangular. A second material in solution is thendeposited in the form of droplets along either side of the ridge. Again,the use of an inkjet print head has been found suitable for thispurpose. The substrate or underlying structure is wetting with respectto the solution of the second material and the ridge of the firstmaterial is non-wetting with respect to the solution of the secondmaterial, that is, it repels the solution of the second material. As aresult, each droplet of the solution of the second material alignsitself along a respective side of the ridge and dries to form a layer ofthe second material along that respective side.

[0032] Finally, the deposition process of the second material can befollowed by a process of removing the ridge of the first material. Toaccomplish this, the substrate can be dipped in a solvent for dissolvingthe first material only. This leaves a layer of the second material onthe substrate formed in two portions, having a narrow gap therebetween.The gap is defined by the width of the ridge of the first material.

[0033] This process can be applied to form a channel for a TFT, whichrequires a narrow gap between a source and a drain. The channel lengthcan be controlled by appropriately selecting the concentration of thefirst material in solution, the drying conditions of the first materialin solution and the conditions of the etching process. A channel lengthof the order 5 μm can be achieved using the above-mentioned patterningmethod.

[0034] Various aspects of the present invention will now be explained inmore detail. FIG. 1 shows the process flow of a patterning methodaccording to the present invention. A solution of a first organic orinorganic material is deposited on to a substrate 1 in the form of adroplet 10 as shown in FIG. 1(a). The function of the first organic orinorganic material is to provide on the substrate 1 a pre-pattern whichhas a particular wetting characteristic with respect to a solution of asecond target material deposited at a later stage. The firstpre-patterning material should have different wetting properties fromthe substrate 1 with respect to the solution of the second targetmaterial.

[0035] When a pattern of the second material is required to be formedwith a small gap therein, the first pre-patterning material should bemore repelling with respect to the solution of the second material thanthe substrate. When a pattern of the second material is required to forma narrow line, the first pre-patterning material should be more wettingthan substrate with respect to the solution of the second material. Forease of explanation, the subsequent description will be primarilydirected to the case where a pattern of the second material is requiredto be formed with a small gap therein. However, this should not betreated as limiting the scope of the present invention.

[0036] Thus, where the second material is PEDOT and the solution thereofis an aqueous solution, a hydrophobic material can be used as the firstpre-patterning material. For example, polystyrene is typical hydrophobicmaterial. However, all chemicals and polymers consisting of non-polargroups show hydrophobic properties and are suited for use as the firstpre-patterning material when a solution of the second target material ina polar solvent is used. In order to achieve a large contrast in thewettability between the first material and the substrate with respect tothe solution of the second material, the substrate 1 should behydrophilic, thereby providing a wetting surface with respect to thepolar solvent. To provide this hydrophilic surface, the substrate 1 may,for example, be exposed to O₂ plasma prior to deposition of the firstpre-patterning material.

[0037] The deposited droplet 10 of the solution of the firstpre-patterning material spreads over the substrate 1 and has a diameterdetermined by the surface tension (surface free energy density) of thedroplet 10 and the substrate 1 respectively and the interfacial tension(interface free energy density) between the droplet 10 and the substrate1, as shown in FIG. 1(b). In the case that a non-polar material ischosen for the first material and for the solvent for the firstmaterial, the surface tension of the solution is in general small (about20˜30 mJ/m²). This results in a small contact angle between the uppersurface of the droplet 10 and the surface of the substrate 1, and alarge diameter of the droplet 10 on the substrate 1. This contact angleis smaller than 30°.

[0038] After the solvent in the droplet 10 has evaporated, thecross-sectional profile of the residue 16 of the first materialdeposited on the substrate is unexpected. This profile is not ahemispheric profile as may be expected, but a ring-shaped profile, asshown in FIGS. 1(c) and 2(a), in which most of the first material isdeposited on the edge and a small amount of the first material isdeposited in the centre region. In other words, a ridge 14 is formedaround the perimeter and a thin layer 12 is formed in the centre region.As will be discussed in greater detail, a small contact angle ispreferable to obtain a significant ridge 14 disposed around theperimeter of the dried droplet 10.

[0039] Where only one droplet 10 is deposited on the substrate 1, theridge 14 is substantially circular. However, if multiple droplets 10 aredeposited with relative displacement between the substrate 1 and adispenser for the droplets 10, such as an inkjet print head 2, anelongated circle such as that shown FIG. 2(a) may be obtained.

[0040] Since the dried droplet 16 includes a thin film 12 in the centreregion, the wettability of the dried droplet 16 with respect to thesolution of the second material is constant over the whole area of drieddroplet 16 of the first pre-patterning material. However, the drieddroplet 16 of the first pre-patterning material is then etched using adry or wet etching process, as shown in FIG. 1(d), to remove the thinfilm 12 in the centre region.

[0041] It should be noted that the chemical properties of the etchant 30selected for use in this etching process can noticeably affect thewetting properties of both the substrate 1 and the dried droplet 16 ofthe first pre-patterning material. Thus, many variations in this etchingprocess are possible. However, to take a simple example, dry etching maybe performed with a plasma of inert gas such as helium, neon, argon,krypton, xenon or nitrogen. The wettability of the substrate 1 and ofthe dried droplet 16 respectively is unaffected by etching with theplasma of inert gas. Thus, if the substrate 1 initially has a wettingsurface and the dried droplet 16 of the first pre-patterning materialinitially has a repelling surface with respect to the solution of thesecond material, these properties are retained after this etchingprocess.

[0042] The parameters of the etching process, including the etchingtime, power density and density of active molecules, are determined soas to obtain a complete removal of the thin film 12 in the centre regionof the dried droplet 16 but to retain a partially etched portion of theridge 14 on the substrate 1, as shown in FIGS. 1(e) and 2(b). Thus, theentire surface of the substrate 1 is exposed except for the remainingnarrow ridge 14, which provides a repelling surface. Thus, a contrastingpattern in wettability, comprising the narrow non-wetting ridge 14 onthe wetting surface of the substrate 1, is defined.

[0043] The width of the ridge 14 can be controlled by the depositionconditions for the solution for the first pre-patterning material, aswill explained below. However, the parameters selected for etching,including choice of the etchant 30, also affect the width of the ridge14. For example, the width of the ridge 14 becomes narrower as etchingtime increases. Thus, it is possible to consistently achieve linesnarrower than 5 μm by optimisation of the deposition conditions and theetching parameters.

[0044] After this etching process, the second target material insolution is deposited along the ridge 14 in the form of one or moresecond droplets 20, as shown in FIG. 1(f), for example using an inkjetprint head 2. The second droplets 20 land on the substrate 1 and spreadon the substrate 1 up to the edge of the ridge 14 of the firstpre-patterning material. The solution of the second material is confinedin the wetting region, on the exposed substrate 1, because the narrowridge 14 repels the solution. Thus, the contrast in wettability betweenthe substrate 1 and the ridge 14 enables alignment of the solution ofthe second material along the edge of the ridge 14. When droplets 20 ofthe solution of the second material are formed on both sides of theridge 14, a narrow gap is formed between them, as shown in FIGS. 1(g)and 2(c).

[0045] As shown in FIG. 2(c), in this example the ridge 14 is formed asan elongated circle and only one portion of the ridge 14 is used forpre-patterning. This is suitable for forming the source and drainelectrodes of a TFT having a short channel length. However, it will beapparent to those skilled in the art that ridges of differing shapes canbe obtained and that other portions or the whole of the ridge 14 can beselected for pre-patterning, depending on the intended application ofthe present invention.

[0046] Upon drying, the second droplets 20 form spaced apart layers 22of the second target material. Spaced apart layers 22 of the secondtarget material, each having a mesa-shaped cross-sectional profile afterdrying, as shown in FIG. 1(h), can be obtained by appropriate selectionof, for example, the concentration of the solution of the secondmaterial, as will be discussed in more detail below. As shown in theFigure, there is a narrow gap between the two layers 22, which cannot beconsistently achieved using conventional inkjet techniques due to thevariations discussed above. However, a consistent, narrow gap can beobtained with the method of the present invention. Moreover, nolithographic techniques are required. The method of the presentinvention can therefore be used at low cost and is suitable for use withlarge substrates, for which lithographic patterning is particularlyexpensive.

[0047] Further, the ridge 14 can be removed, for example by bathing thesubstrate 1 in a solvent for the first pre-patterning material. Thesecond target material is not adversely affected by such a strippingprocess since it is insoluble in the solvent for the first material.FIGS. 1(i) and 2(d) show a complete pattern of the second targetmaterial after removal of the ridge 14.

[0048] The shape of a dried droplet 16 of the first pre-patterningmaterial and of a layer 22 of the second target material is affected bya number of factors, including the properties of the solutions of therespective materials and the drying conditions, as will now beexplained. When a droplet is deposited on a substrate, there are atleast three deposition modes, as shown in FIG. 3, each of which resultsin a different cross-sectional profile of the remaining film of soluteon the substrate after the solvent has evaporated. The first depositionmode may be termed shrink type deposition and is shown in FIG. 3(a). Inthis mode, the lateral size or diameter of the deposited film 42 ofsolute after drying is smaller than the diameter of liquid droplet 40 ofsolution when first deposited on the substrate, the outline of which isshown as the outermost dotted line. This means the droplet 40 has shrunkduring drying. Shrink type deposition occurs when the contact angle of adroplet 40 is relatively large (for example, greater than 40°) and whenthe solution does not wet the surface of the substrate well. Thedeposition of a droplet of an aqueous solution onto a hydrophobicsurface is an example of shrink type deposition, because of the largecontact angle of the droplet with the surface and the non-wettingproperties of water on the surface. Thus, when a water-based PEDOTsolution is deposited onto a polystyrene film, this shrink typedeposition is observed.

[0049] The second mode is mesa type deposition, which is used fordeposition of the second target material in the present example and isshown in FIG. 3(b). In this mode, a small amount of shrinkage of adroplet 20 during drying can also be observed. However, in contrast withshrink type deposition, deposition of the solute occurs duringshrinking. As a result, the deposited film 22 has a mesa-shaped profileand a diameter almost the same size as the initial diameter of thedroplet 20 of solution. Mesa type deposition occurs when the surfacetension of the solution is high (for example, greater than 30 mJ/m²) andwhen the solution can wet the surface of the substrate. Mesa typedeposition may occur when an aqueous solution is deposited onto ahydrophilic surface or an inorganic substrate, since water has a highsurface tension and good wetting properties with respect to ahydrophilic surface. When a water-based PEDOT solution is deposited ontoa glass substrate, for example, mesa type deposition may be observed.

[0050] The last mode is ring type deposition, which is used fordeposition of the first pre-patterning material in the present exampleand is shown in FIG. 3(c). The film resulting from this type ofdeposition may be equated with the film of residue left after coffee isspilt on a solid surface. In ring type deposition, as, discussed above,most of the solute is deposited at the edge of the droplet 10, therebyproviding a ridge 14 of the solute around the edge and a thin film 12 ofthe solute in the centre region. Ring type deposition occurs when thecontact angle is relatively small (for example, less than 30° andpreferably less than 20°) and when wetting of the solution on thesubstrate is fairly good. In this case, the contact line of the dropleton the substrate is pinned and does not move during drying. This meansthe contact angle decreases as the droplet 10 dries. This can occur whenthe surface tension of the solution is low (for example, less than 30mJ/m²).

[0051] An internal fluid flow 50 is caused in the pinned droplet 10 byenhanced evaporation and volume effects 52 and 54 respectively, asillustrated schematically in FIG. 4. The enhanced evaporation effect 52is caused by a difference in the speed of evaporation of the solvent atthe edge and centre regions of the droplet 10 respectively. A higherevaporation rate is observed in the edge region than in the centreregion because solvent molecules escape more easily from the edgeregion. An internal flow 50 of fluid within the droplet 10 takes placeso as to compensate this difference. However, since the contact line ispinned and the volume of the droplet is decreasing, the shape of thedroplet changes. Thus, the volume change in the edge region is smallerthan that in the centre region, as shown in FIG. 3(c) or FIG. 4. Thisdifference in the change of volume at the edge and centre regionsrespectively also causes internal flow. Accordingly, due to these twoeffects, an internal flow from the centre to the edge occurs in adroplet drying on a substrate in the ring type deposition mode. Theviscosity of the fluid has an influencing effect on the internal flow.Therefore, an efficient (fast) flow occurs in a droplet of low viscosityfluid. When the viscosity of the fluid (solution) is less than about 10cps, a narrow ridge is obtained. Viscosity of lower than about 4 cps isespecially suitable to form a narrow ridge. The viscosity of thesolution depends on the concentration of the solute (solid contents),the viscosity of solvent or temperature. If the concentration of thesolute (organic polymer) in the solution is reduced, this results inlower concentration and hence an increase in the velocity of internalflow from the centre to the edge of a drying droplet. The use of lowviscosity solvent leads to such an increase in the internal flow aswell, which results in a narrow ridge (edge). When polymer is used asthe solute, the molecular weight of the polymer and the conformation ofthe polymer in the solution affect the viscosity of the solution. Inorder to obtain low viscosity, a molecular weight of less than about100,000 is preferable.

[0052] Furthermore, the drying speed of the solution is an importantparameter, because the speed of the internal flow is determined by thedrying speed. If the solution is dried faster, this results in anincrease in the velocity of the internal flow and hence the narrowing ofthe ridge. The drying speed is enhanced by using a volatile solvent, byproviding a gas flow (which may be heated) over the solution, or byheating the solution droplet. When the process is carried out at a roomtemperature, the boiling point of the solvent is preferably lower thanabout 160° C. and more preferably lower than about 120° C. to obtainhigh drying speed. Heating the solution droplet is especially effectiveto obtain a narrow ridge, because it is effective in two ways: theenhancement of the drying speed and the lowering of viscosity. For thispurpose, the droplets of the solution can be deposited on a substratewhich is heated from about 40° C. to about 150° C.

[0053] This internal flow 50 carries solute in the solution from thecentre to the edge. As a result, there is an enhanced deposition of thesolute in the edge region and a thin layer is formed in the centreregion. The velocity of the flow 50 affects the cross sectional profileof the film and depends on the contact angle, the evaporation speed andthe viscosity of the sessile droplet. The velocity of the flow 50increases with decreasing of a contact angle, so a contact angle of lessthan 30° is desirable to achieve a ridge 14 having a narrow width. Anincreased evaporation speed also increases the velocity of the flow 50,so a high evaporation speed narrows the ridge width. The ridge width cantherefore be controlled by controlling the concentration of the organicpolymer in the deposited solution. The evaporation speed and henceinternal flow can also be increased by using a low boiling pointsolvent, by raising the temperature during drying and by loweringsolvent vapour pressure around the droplet. A stream of gas (forexample, dry air, nitrogen or argon) passed over a substrate effectivelylowers this pressure, resulting in narrowing of the ridge 14. When thereis a uni-directional gas flow over the substrate, this causes asymmetricdeposition at the edge region. Since the gas upstream is clean, thesolvent vapour pressure is lower upstream than downstream, internal flow50 in the droplet 10 is asymmetric and a larger amount of depositionoccurs at the upstream edge than at the downstream edge. This phenomenonis also useful for controlling the height and the width of the ridge 14and especially for obtaining a high and narrow ridge 14.

[0054] The viscosity of the solution also plays an important role. Ifthe solution were to have zero viscosity, all the solute would becarried to the edge due to the very high velocity of internal flow 50 atthe final stage of drying. In fact, the viscosity increases as dryingproceeds due to an increase in the concentration of the solution. Thisincrease of viscosity lowers the speed of internal flow 50 andeventually stops it. This behaviour determines the thickness of the film12 at the centre region and the height and the width of the ridge 14.When a thin solution having a low viscosity is used, a very thin film 12is obtained at the centre region and high and narrow ridge 14 isobtained at the edge. As described above, the viscosity depends not onlyon the concentration of the solution but also on the combination ofsolute and solvent and the molecular weight of the solute. Theseparameters can be optimised so as to obtain low viscosity even afterdrying has progressed. In general, however, a solution having aviscosity lower than about 4 cps is preferable and a solution having aviscosity lower than 2 cps is more preferable still. When polymers areused for the solute, such viscosities can generally be attained withconcentrations less than about 3% or, preferably, 1%.

[0055] Dry etching with the plasma of an inert gas has been previouslyexplained, but other etching techniques, having various other effects,may also be used.

[0056] Firstly, dry etching may be performed using oxygen plasma, whichhas the effect of providing a surface with wetting properties,especially with respect to polar solutions. In other words, the surfaceof both the substrate 1 and the ridge 14 may be turned hydrophilic bydry etching using oxygen plasma. This is due to the oxidation of thesurface or the termination of molecules at the surface by hydroxylgroups formed by a reaction of radical oxygen molecules on the surfaceand water molecules in air. Since the polarity of oxygen or a hydroxylgroup may induce hydrophilic properties in the surface of both thesubstrate 1 and the ridge 14, the wettability contrast between the ridgeand the substrate is lost by oxygen plasma etching for many combinationsof substrate material and first pre-patterning material.

[0057]FIG. 5(a) shows the case where the surface of both the ridge 14and the substrate 1 are terminated with hydroxyl groups (—OH) andtherefore both have hydrophilic properties, irrespective of whethereither the substrate or the first unetched pre-patterning material wasoriginally hydrophobic. However, it has been found that the regionmodified by oxygen plasma remains at the surface and that the originalcontrast in wettability can be recovered by eliminating the surfaceregion of either the ridge of the substrate. In this example, thesurface of the first pre-patterned material, that is, the ridge 14, isremoved. This can be achieved by etching.

[0058] As discussed above, non-polar polymers are suitable for use asthe first pre-patterning material where the second target material isPEDOT deposited in a water-based solution. Such non-polar polymersinclude polymers consisting of aromatic groups and alkyl groups. Forexample, polystyrene and polyethylene are typical non-polar polymers.

[0059] In order to etch the surface region of the first pre-patterningmaterial, a solvent for dissolving the first material, diluted withother solvents that do not dissolve the first material, can be used asthe etchant. Such a diluted solvent dissolves the first material slowly,so it is possible to etch only the surface region of the ridge 14,thereby removing the hydroxyl groups, as shown in FIG. 5(b). Whenpolystyrene is used as the first pre-patterning material, aromaticand/or chlorinated hydrocarbons diluted by alcohols or aliphatichydrocarbons can be used as the etchant. Toluene diluted by isopropanolis a typical example of such an etchant.

[0060] In contrast, the substrate 1 is not dissolved in an etchantcomprising such a diluted solvent and the hydroxyl groups are thereforeretained on the surface of the substrate 1, as shown in FIG. 5(b). Thesurface of the substrate 1 thus remains hydrophilic. As a result, thepattern of contrasting wettability on the oxygen plasma etched substrate1 can be recovered after slight etching with such a diluted solvent.Thus, self-alignment of the second target material with the firstpre-patterning material can be achieved after this treatment.

[0061] In general, however, non-polar materials do not adhere or stickto hydrophilic surfaces very well. This has the result that a firstpre-patterning material comprising a non-polar polymer may be lifted offthe hydrophilic surface of the substrate 1 during this slight etchingwith a diluted solvent. In order to improve the adhesive characteristicsof the first pre-patterning material with respect to the surface of thesubstrate 1, primers can be used to change the surface properties of thesubstrate 1 from hydrophilic to hydrophobic. Hexamethyldisilazane (HMDS)is a typical primer suitable for this purpose. For example, thesubstrate 1 can be pre-treated, prior to deposition of the firstmaterial, using HMDS in vapour or liquid form at 70 to 140° C. Thesurface of the pre-treated substrate 1 is terminated with hexamethylgroups, which provide hydrophobic properties. Good adhesion of anon-polar first material on such a pre-treated substrate 1 can thereforebe obtained.

[0062] An additional, advantageous effect of this pre-treatment is thata narrower ridge 14 of the first material can be obtained, since thecontact angle of the solution of the first material with the substrate 1is reduced. Moreover, hexamethyl groups on the surface of thepre-treated substrate 1 are easily removed by oxygen plasma etching,discussed above. Thus, the original hydrophilic surface properties ofthe untreated substrate 1 are recovered or improved by oxygen plasmaetching so the pattern of contrasting wettability between the substrate1 and the ridge 14 can still be obtained by slight etching with adiluted solvent.

[0063] An alternative method for recovering the wettability contrastafter dry etching is to expose the oxygen plasma etched substrate 1 to avapour of a solvent for dissolving the first pre-patterning material.The solvent vapour is absorbed in the pre-patterning material and allowsthe material to reorient into a more stable, hydrophobic form. Due tothis reorientation, hydroxyl groups at the surface of the ridge 14 arereplaced by the first material, which is more stable. As a result, thesurface of the ridge 14 recovers its original hydrophobic properties andthe surface of the substrate 1 retains its hydrophilic state. Thisexposure method uses a vapour phase and is therefore a dry process. Itis therefore unnecessary to dip the substrate 1 into a liquid, which istime-consuming, dirty and expensive. When polystyrene is used for thefirst pre-patterning material for example, aromatic and/or chlorinatedhydrocarbons can be used for the solvent vapour. Toluene or xylene is atypical aromatic hydrocarbon solvent.

[0064] As a further alternative, plasma etching with a mixture offluorinated carbons and another gas can be used and has been found to bean especially effective method for achieving a high contrast inwettability. When an inorganic substrate 1 having an organicpre-patterning material, ridge 14, thereon is plasma etched using amixture of fluorinated carbons and oxygen, the surface of organicpre-patterning material is fluorinated since hydrogen atoms binding tocarbon atoms therein are replaced by fluorine, as shown in FIG. 6(a).This fluorinated surface is non-wetting with respect to both polarsolutions and non-polar solutions. On the other hand, as shown in FIG.6(a), the surface of the inorganic substrate 1 is terminated by hydroxylgroups due to the effect of oxygen plasma. The surface of the substrate1 is therefore wetting, especially with respect to polar solutions.

[0065] In contrast, when an inorganic substrate 1 with a ridge 14 oforganic pre-patterning material thereon is etched with a mixture offluorinated carbons and inert gases (Ar, N₂), the surface of the ridge14 is fluorinated, while the surface of the substrate 1 is maintained.This provides a good contrast in wettability, especially with respect tonon-polar solutions.

[0066] In order to pattern source and drain electrodes in TFTs, thesolution of the second target material, for example PEDOT water-basedsolution, may be deposited twice, once for each of the source and drain.However, the fluorinated surface of the ridge 14 shown in FIGS. 6 and7(a) is sufficiently non-wetting that droplets 20 of PEDOT solution canbe deposited only once, directly onto the ridge 14, as shown in FIG.7(b). The solution is repelled by the ridge 14 and is divided by theridge to lie on two portions on the hydrophilic surface of the substrate1, one on either side of the ridge 14, as shown in FIG. 7(c). Depositionof this second target material only occurs on the substrate 1, withoutthe formation of a bridge of the second target material over ridge 14,which would cause a short circuit between the two portions. Thus, thesecond target material can be very easily deposited with highreliability.

[0067] A TFT can be fabricated using the method of the present inventiondescribed above. The structure of a printable TFT is shown in FIG. 9, inwhich a glass or plastic substrate 1 may be used. Flexible devices maybe obtained using a plastic substrate. Polyimide, polyethylenenaphthalate, polyphenylene sulfide, polyetherkiton or polyethersulfoneare all suitable for use as a plastic substrate in terms of resistanceto heat and solvents during later processing to fabricate the deviceusing conventional techniques. When the temperature of such conventionalprocessing is less than 100° C., less expensive materials such aspolyethylene terephthalate, polymethyl methacrylate or polycarbonate canbe used. On the substrate, an alignment layer 66 may optionally beformed in the event that alignment of molecules or polymer chains isrequired in the semiconductor. Rubbed polyimide is a typical materialcommonly used for such an alignment layer. Such rubbed polyimide layersmay be obtained by rubbing cured polyimide layers with textured fabricin a direction parallel to a channel.

[0068] On this alignment layer 66, the first pre-patterning material isdeposited in the form of strips 16 by an inkjet print head, as shown inFIG. 10(a). These pre-pattern strips 16 are non-wetting with respect tothe solution of the second target material. In this example, a PEDOTwater-based solution is used as the solution of the second material, sothe first pre-patterning material is hydrophobic.

[0069] Many kinds of hydrocarbon or fluorinated carbon are suitable forthis purpose. Among these, non-polar polymers are appropriate materialswith respect to the formation of uniform films. Polystyrene is one suchnon-polar polymer and a solution of 0.1-4% polystyrene in toluene orxylene can be used as the solution of the first pre-patterning material,for example. The dried deposited strips 16 have a thin thickness in thecentre region and a ridge around the edge. As discussed above, thecross-sectional profile depends on the drying conditions. Since narrowand high ridges can be obtained by quick drying, a stream of N₂ ispassed over the substrate 1, as shown in FIG. 10(a) and/or the substrate1 is heated.

[0070] In order to etch the thin film at the centre region, the sampleis etched using oxygen plasma. Since the film is thin, in the range 5 to20 nm, the etching time is short. For example, the etching time for a 20nm thick film is about 5 minutes where the oxygen pressure is 2.5 mbarand the RF power density is 4000 W/m². The etched thickness (depth) isapproximately proportional to the product of the etching time and the RFpower density. The minimum etched thickness required is the thickness ofthe film at the centre region and with further etching the width of theridge can be narrowed. This further etching has reasonably goodreproducibility due to the large difference in thickness of the thinfilm at the centre and the ridge: the thickness of ridge can becontrolled to be generally larger than 300 run, even when the thicknessof the centre deposition is smaller than 20 nm. This large contrast inthickness makes it possible to accurately control the width of theridge. With minimum etching to remove the thin film in the centreregion, a ridge as narrow as 10 to 30 μm is obtainable, and with furtheretching 3 to 15 μm width ridges can be easily achieved over the wholesubstrate.

[0071] At this stage, the surface of both the substrate 1 and the ridgesare hydrophilic, so the wettability contrast needs to be recovered byselective etching or reorientation of the ridges of first pre-patterningmaterial. A mixture of xylene and isopropanol, in the ratio 5:95 forexample, can be used for the selective etching. This mixture does notdissolve the rubbed polyimide alignment layer 66, but removes thesurface region of the ridges of pre-patterning material. After theselective etching process, elongated ringed ridges 14 of thepre-patterning material are obtained, as shown in FIG. 10(b).

[0072] Each elongated ringed ridge may be used to fabricate more thanone TFT. As illustrated in FIG. 10(c), droplets 20 of the second targetmaterial, forming the source 60 and drain 62 of the TFT (see FIG. 9),are deposited so as to form a plurality of transistors along one ridge.PEDOT water-based solution is used as the second target material. Thiswets the surface of the rubbed polyimide alignment layer 66, but it isrepelled by the polystyrene pre-patterning ridge 14. These propertiescause a narrow gap to be formed in the second target material, along theridge 14.

[0073] This step can be followed by removal of the ridge 14 with asolvent for the first material, as shown in FIG. 10(d). A semiconductorlayer 68 and an insulating layer 70 are then deposited on the source 60and drain 62 electrodes in order to fabricate the structure shown inFIG. 9. Several techniques for depositing these layers are known,including evaporation, spin-coating, screen-printing andinkjet-printing. Gate electrodes 64 and interconnections 90 are thendeposited on the insulating layer 70, preferably by ejection of a PEDOTwater-based solution by an inkjet print head.

[0074] Alignment of the gate electrode 64 with the channel is required,so an inkjet head and a substrate holder should be positioned so as toachieve this alignment. Since inkjet printing is a non-contact printingtechnique, such positioning is relatively easy and accurate compared toother contact printing techniques.

[0075] Thus, a TFT array such as that shown in FIG. 10(e) can beobtained. Even though these TFTs have a short channel (less than 30 μm),they are achieved simply using an inkjet printing technique. The methodof the present invention does not require photolithography for thepre-patterning and allows production of TFT arrays with a very lowmanufacturing cost. The method of the present invention also enables thefabrication of very large, flexible devices, which can only be obtainedusing conventional photolithographic techniques with considerabledifficulty.

[0076] A method of patterning a substrate and/or a method of forming athin film transistor according to the present invention can bepreferably applied to manufacture of an electro-optical device, asemiconductor device and other electronic devices. In other words, a TFTarray manufactured by the method according to the present invention canbe preferably used in some kinds of electro-optical devices. Theelectro-optical devices preferably include a liquid crystal device, anorganic electoluminescent device, an inorganic electroluminescentdevice, a field-emission device (FED), a plasma device, anelectrophoretic device, and other display devices. These devices can bepreferably applied to a display apparatus. In particular, such a TFTarray can be more preferably applied to pixel circuits and/or drivercircuits formed in an active matrix substrate used in the above displaydevices.

[0077]FIG. 11 is a block diagram illustrating an active matrix typedisplay device (or apparatus) incorporating electro-optical elementssuch as organic electroluminescent elements as a preferred example ofthe electro-optical devices. In the display device 200 shown in thisfigure, a plurality of scanning lines “gate”, a plurality of data lines“sig” extending in a direction that intersects the direction in whichthe scanning lines “gate” extend, a plurality of common power supplylines “corn” extending substantially parallel to the data lines “sig”,and a plurality of pixels 201 located at the intersections of the datalines “sig” and the scanning lines “gate” which are formed above asubstrate.

[0078] Each pixel 201 comprises a first TFT 202, to which a scanningsignal is supplied to the gate electrode through the scanning gate, aholding capacitor “cap” which holds an image signal supplied from thedata line “sig” via the first TFT 202, a second TFT 203 in which theimage signal held by the holding capacitor “cap” is supplied to the gateelectrode (a second gate electrode), and an electro-optical element 204such as an electroluminescent element (indicated as a resistance) intowhich the driving current flows from the common power supply line “com”when the element 204 is electrically connected to the common powersupply line “corn” through the second TFT 203. The scanning lines “gate”are connected to a first driver circuit 205 and the data lines “sig” areconnected to a second driver circuit 206. At least one of the firstcircuit 205 and the second circuit 205 can be preferably formed abovethe substrate above which the first TFTs 202 and the second TFTs 203 areformed. The TFT array(s) manufactured by the methods according to thepresent invention can be preferably applied to at least one of an arrayof the first TFTs 202 and the second TFTs 203, the first driver circuit205, and the second driver circuit 206.

[0079] The present invention may therefore be used to fabricate displaysand other devices which are to be incorporated in many types ofequipment such as mobile displays e.g. mobile phones, laptop personalcomputers, DVD players, cameras, field equipment; portable displays suchas desktop computers, CCTV or photo albums; instrument panels such asvehicle or aircraft instrument panels; or industrial displays such ascontrol room equipment displays. In other words, an electro-opticaldevice or display to which the TFT array(s) manufactured by the methodsaccording to the present invention is (are) applied as noted above canbe incorporated in the many types of equipment, as exemplified above.

[0080] Various electronic apparatuses using electro-optical displaydevices fabricated in accordance with the present invention will now bedescribed.

[0081] <1: Mobile Computer>

[0082] An example in which the display device fabricated in accordancewith one of the above embodiments is applied to a mobile personalcomputer will now be described.

[0083]FIG. 12 is an isometric view illustrating the configuration ofthis personal computer. In the drawing, the personal computer 1100 isprovided with a body 1104 including a keyboard 1102 and a display unit1106. The display unit 1106 is implemented using a display panelfabricated according to the present invention, as described above.

[0084] <2: Portable Phone>

[0085] Next, an example in which the display device is applied to adisplay section of a portable phone will be described. FIG. 13 is anisometric view illustrating the configuration of the portable phone. Inthe drawing, the portable phone 1200 is provided with a plurality ofoperation keys 1202, an earpiece 1204, a mouthpiece 1206, and a displaypanel 100. This display panel 100 is implemented using a display deviceaccording to the present invention, as described above.

[0086] <3: Digital Still Camera>

[0087] Next, a digital still camera using an OEL display device as afinder will be described. FIG. 14 is an isometric view illustrating theconfiguration of the digital still camera and the connection to externaldevices in brief.

[0088] Typical cameras use sensitized films having light sensitivecoatings and record optical images of objects by causing a chemicalchange in the light sensitive coatings, whereas the digital still camera1300 generates imaging signals from the optical image of an object byphotoelectric conversion using, for example, a charge coupled device(CCD). The digital still camera 1300 is provided with an OEL element 100at the back face of a case 1302 to perform display based on the imagingsignals from the CCD. Thus, the display panel 100 functions as a finderfor displaying the object. A photo acceptance unit 1304 includingoptical lenses and the CCD is provided at the front side (behind in thedrawing) of the case 1302.

[0089] When a cameraman determines the object image displayed in the OELelement panel 100 and releases the shutter, the image signals from theCCD are transmitted and stored to memories in a circuit board 1308. Inthe digital still camera 1300, video signal output terminals 1312 andinput/output terminals 1314 for data communication are provided on aside of the case 1302. As shown in the drawing, a television monitor1430 and a personal computer 1440 are connected to the video signalterminals 1312 and the input/output terminals 1314, respectively, ifnecessary. The imaging signals stored in the memories of the circuitboard 1308 are output to the television monitor 1430 and the personalcomputer 1440, by a given operation.

[0090] Examples of electronic apparatuses, other than the personalcomputer shown in FIG. 12, the portable phone shown in FIG. 13, and thedigital still camera shown in FIG. 14, include OEL element televisionsets, view-finder-type and monitoring-type video tape recorders, carnavigation systems, pagers, electronic notebooks, portable calculators,word processors, workstations, TV telephones, point-of-sales system(POS) terminals, and devices provided with touch panels. Of course, theabove OEL device can be applied not only to display sections of theseelectronic apparatuses but also to any other form of apparatus whichincorporates a display section.

[0091] Furthermore, the display devices fabricated in accordance withthe present invention are also suitable for a screen-type large areatelevision which is very thin, flexible and light in weight. It ispossible therefore to paste or hang such large area television on awall. The flexible television can, if required, be conveniently rolledup when it is not used.

[0092] The aforegoing description has been given by way of example onlyand it will be appreciated by a person skilled in the art thatmodifications can be made without departing from the scope of thepresent invention. For example, a person skilled in the art willappreciate that a wide variety and various combinations of substrates,pre-patterning materials and target materials, individually andtogether, can be selected. In addition, it will be appreciated that avariety of shapes, sizes and patterns of pre-patterning material can beused. It will, for example, be further appreciated that selected linesof a second target material, bounded on either side, can be providedeither using a single line of a first pre-patterning material, which iswetting with respect to the second target material, or using two linesof a first pre-patterning material, which is non-wetting with respect tothe second target material.

1. A method of patterning a substrate, the method comprising: depositinga first material in solution on the substrate, wherein the solution ofthe first material is selected such that a profile of the first materialdried on the substrate includes a ridge; etching for removing portionsof the first material such that the ridge of the first material remainson the substrate; and depositing a second material in solution on oradjacent the ridge of the first material.
 2. A method according to claim1, wherein the wetting characteristic of the etched ridge of the firstmaterial with respect to the solution of the second material isdifferent to that of the substrate.
 3. A method according to claim 1 orclaim 2, wherein the wetting characteristic of the first material withrespect to the solution of the second material is different to that ofthe substrate.
 4. A method according to any one of the preceding claims,wherein said etching is dry etching using an inert gas.
 5. A methodaccording to any one of claims 1 to claim 3, wherein said etchingfurther comprises adjusting the wetting characteristics of the firstmaterial and of the substrate with respect to the solution of the secondmaterial.
 6. A method according to claim 5, wherein said etchingcomprises a first step of etching for removing portions of the firstmaterial, thereby adjusting the wetting characteristics of the firstmaterial and the substrate, and a second step of adjusting the wettingcharacteristics of the first material.
 7. A method according to claim 6,wherein the first step of etching comprises dry etching.
 8. A methodaccording to claim 7, wherein oxygen plasma is used for the first stepof etching.
 9. A method according to any one of claims 6 to 8, furthercomprising, before said depositing the first material in solution,improving the wetting characteristics of the substrate with respect tothe first material.
 10. A method according to any one of claims 6 to 9,wherein a diluted solvent for the first material is used for the secondstep.
 11. A method according to any one of claims 6 to 9, wherein vapourof a solvent for the first material is used for the second step.
 12. Amethod according to claim 10 or claim 11, wherein the first materialcomprises a non-polar polymer and the solvent comprises aromatic and/orchlorinated hydrocarbons.
 13. A method according to claim 12 wherein thesolvent comprises toluene and/or xylene.
 14. A method according to claim12 or claim 13, wherein alcohols and/or aliphatic hydrocarbons are alsoused for the second step.
 15. A method according to claim 14, whereinsaid alcohols and/or aliphatic hydrocarbons comprise isapropanol.
 16. Amethod according to claim 5, wherein said etching comprises plasmaetching using a mixture of fluorinated carbon atoms and another gas. 17.A method according to claim 16, wherein said another gas comprisesoxygen.
 18. A method according to claim 16, wherein said another gascomprises an inert gas.
 19. A method according to any one of thepreceding claims, wherein the substrate and the solution of the firstmaterial are selected such that said ridge is formed along the perimeterof the deposited first material.
 20. A method according to claim 19,wherein the perimeter includes at least one substantially straightportion.
 21. A method according to claim 20, wherein the perimetercomprises an elongated oval or circle having two substantially straightportions.
 22. A method according to claim 20, wherein the secondmaterial is deposited on or adjacent said at least one straight portiononly.
 23. A method according to claim 21, wherein the second material isdeposited on or adjacent both the substantially straight portions.
 24. Amethod according to any one of the preceding claims wherein theconcentration of the first material in solution is selectivelycontrolled so as to selectively control the profile of the firstmaterial dried on the substrate.
 25. A method according to any one ofthe preceding claims comprising selecting the viscosity for a solventfor the first material so as to selectively control the profile of thefirst material dried on the substrate.
 26. A method according to claim25 wherein the viscosity is selected to be less than about 10 cps andpreferably less than 4 cps.
 27. A method according to claim 25 or 26comprising selecting the volatility of the solvent so as to selectivelycontrol the profile of the first material dried on the substrate.
 28. Amethod according to claim 27 wherein the solvent is selected to have aboiling point of less than about 160° C. and preferably less than about120° C.
 29. A method according to any one of the preceding claims,further comprising, after said depositing the first material in solutionand before said etching, passing a stream of gas over the substrate fordrying the solution of the first material.
 30. A method according toclaim 29 wherein the stream of gas is provided as a unidirectionalstream.
 31. A method according to claim 29 or 30 wherein the stream ofgas is heated.
 32. A method according to any one of the precedingclaims, further comprising, after said depositing the first material insolution and before said etching, heating the substrate for drying thesolution of the first material.
 33. A method according to claim 32wherein the substrate is heated to a temperature within a range of fromabout 40° C. to about 150° C.
 34. A method according to any one of thepreceding claims, wherein, after etching, the ridge is more wettablethan the substrate with respect to the solution of the second materialsuch that the second material dries on the ridge.
 35. A method accordingto any one of claims 1 to 33, wherein, after etching, the substrate ismore wettable than the ridge with respect to the solution of the secondmaterial, such that the second material dries substantially on thesubstrate only, adjacent the ridge.
 36. A method according to claim 35,wherein the solution of the second material is deposited adjacent eitherside of the ridge.
 37. A method according to claim 36, wherein thesolution of the second material is deposited on the ridge such that thesecond material dries adjacent either side of the ridge.
 38. A methodaccording to any one of claims 35 to 37, wherein the solution of thesecond material is selected such that a profile of the second materialdried on the substrate is mesa-shaped.
 39. A method according to any oneof claims 35 to 38, wherein the first material is a non-polar polymerand the solution of the second material is a water-based PEDOT solution.40. A method according to claim 39 wherein the non-polar polymer isselected to have a molecular weight of less than about 100,000.
 41. Amethod according to claim 39 or 40, wherein the first material comprisesaromatic and/or alkyl groups and does not comprise hydroxyl, diazo orany acid group.
 42. A method according to claim 41, wherein the firstmaterial is polystyrene or polyethylene.
 43. A method according to anyone of claims 35 to 42, further comprising a step of removing the ridgefrom the substrate.
 44. A method of forming a thin film transistorcomprising a method according to claim 43, wherein the second materialforms a source and a drain with a gap therebetween, the method furthercomprising: depositing a semiconductor layer over the second materialand the substrate; depositing an insulating layer over the semiconductorlayer; and providing a gate electrode over the gap.
 45. A methodaccording to claim 44, wherein the substrate comprises an alignmentlayer on which the first material is deposited.
 46. A method accordingto claim 45, wherein the alignment layer is brushed polyimide.
 47. Amethod according to any one of claims 44 to 46, wherein at least one ofthe semiconductor layer, the insulating layer and the gate electrode isdeposited using an inkjet print head.
 48. A method according to claim47, wherein the inkjet print head is moved relative to the substrate fordepositing said at least one of the semiconductor layer, the insulatinglayer and the gate electrode.
 49. A method according to any one of thepreceding claims, wherein at least one of the solution of the firstmaterial and the solution of the second material is deposited using aninkjet print head.
 50. A method according to claim 49, wherein theinkjet print head is moved relative to the substrate for depositing saidat least one of the solution of the first material and the solution ofthe second material
 51. A method according to any one of the precedingclaims, wherein the substrate is selected to comprise glass or plastic.52. A method according to any one of claims 1 to 51, wherein thesubstrate is selected to comprise a flexible material.
 53. A devicefabricated using a method according to any one of the preceding claims.54. A device according to claim 53, wherein the device comprises anelectro-optic device.
 55. A device according to claim 53, wherein thedevice comprises a thin film transistor.
 56. A device according to claim53 or 55, wherein the device comprises an array of thin filmtransistors.
 57. A device according to any one of claims 53 to 56,wherein the device is a display device.
 58. An organic thin filmtransistor, comprising a channel between a source and a drain, whereinthe channel has a length of 25 μm or less.
 59. An organic thin filmtransistor according to claim 58, wherein the length of the channel is10 μm or less.
 60. An array of thin film transistors according to claim58 or claim
 59. 61. Electronic apparatus comprising an electro-opticdevice according to claim 54, an array of thin film transistorsaccording to claim 56, a display device according to claim 57 or anorganic thin film transistor according to claim 58 or 59.